A thermoplastic elastomer has been conventionally widely used as an automobile part such as a bumper, etc. This thermoplastic elastomer has both of thermoplastic properties and elastic properties, and can be molded, by injection molding, extrusion molding, etc, into an article which is excellent in heat resistance, tensile properties, weatherability, flexibility and elasticity.
Japanese Patent Publication No. 34210/1978 discloses a thermoplastic elastomer in which 60 to 80 parts by weight of a monoolefin copolymer rubber and 40 to 20 parts by weight of polyolefin plastic are dynamically and partially cured. Japanese Patent Publication No. 21021/1978 discloses a thermoplastic elastomer comprising (a) a partially crosslinked copolymer rubber composed of an ethylene-propylene-non-conjugated polyene copolymer rubber and having a gel content of 30 to 90% by weight and (b) a polyolefin resin. Further, Japanese Patent Publication No. 18448/1980 discloses a thermoplastic elastomer in which an ethylene-propylene copolymer rubber and a polyolefin resin are dynamically and either partially or completely crosslinked. Meanwhile, Japanese Laid-Open Patent Publication No. 187412/1983 discloses a crosslinked block copolymer which is derived from an olefinic block copolymer containing 50 to 70 parts by weight of a block (A) having a propylene content of 100 to 60% by weight and selected from a propylene homopolymer block and a binary propylene-ethylene or propylene-C.sub.4 -C.sub.12 .alpha.-olefin random copolymer block and 30 to 50 parts by weight of a binary ethylene-propylene random copolymer block (B) having an ethylene content of 30 to 85% by weight, and which has characteristic features in a specific content of a hot xylene-insoluble component and specific fluidity.
Further, Japanese Laid-Open Patent Publications Nos. 165414/1988, 165115/1988 and 161516/1988 and U.S. Pat. No. 4,454,306 disclose a process for producing a crosslinked olefinic block copolymer, which comprises kneading and crosslinking an olefin block copolymer containing a propylene homopolymer block (A) prepared in the presence of a specific Ziegler catalyst, a binary propylene ethylene random copolymer block (B) and a binary propylene ethylene random copolymer block (C) together with an organic peroxide, a divinyl compound and an antioxidant at a temperature of not more than 230.degree. C. Japanese Laid-Open Patent Publication No. 21731/1973 discloses a process for improving a block copolymer in processability, which comprises mixing an organic peroxide with a block copolymer containing 3 to 30% by weight of a copolymer portion which is composed mainly of ethylene and contains not more than 70% by weight of an other .alpha.-olefin and 97 to 70% by weight of a polymer portion composed mainly of propylene, and subjecting the resultant mixture to heat treatment at a temperature between 180.degree. C. and 270.degree. C.
Further, Japanese Laid-Open Patent Publication No. 90007/1982 discloses a process which comprises mixing 100 parts by weight of a block copolymer or a copolymer composition containing a polypropylene polymer portion and an ethylene propylene copolymer portion having an ethylene unit content of 20 to 80% by weight, in which the intrinsic viscosity of the polypropylene polymer portion is 1.4 to 3 dl/g, the content of this portion is not less than 70% by weight, the intrinsic viscosity of the ethylene.propylene copolymer portion is 4 dl/g, the content of this portion is not less than 2% by weight and the total ethylene unit content is not less than 5% by weight, 0.03 to 10 parts by weight of an .alpha., .beta.-unsaturated carboxylic acid or a derivative thereof, 0.005 to 1 part by weight of a radical reaction initiator and not less than 0.03 part by weight of water, and kneading the resultant mixture while it is melted. The above Publication also discloses that according to this process, there can be obtained a modified polypropylene resin molding material which has a melt flow index of 0.2 to 50 g/10 minutes and a grafted .alpha., .beta.-unsaturated carboxylic acid or its derivative graft in an amount of 0.03 to 5% by weight and which substantially does not contain a gel component. It is described that this modified polypropylene resin molding material substantially does not contain a gel component and that the gel percentage of the gel component insoluble in boiling xylene is hence not more than 2% by weight. That is, the process for producing a modified polypropylene resin molding material, disclosed in the above Publication, is solely intended for graft modification, and it is not intended to carry out the crosslinking at the same time.
The present inventors have made studies to produce a thermoplastic elastomer directly and dynamically by heat-treating polymer particles as an economical process, and found that when polymer particles having a specific composition with regard to crystallizability are used as polymer particles, there can be obtained a molded article which has remarkable uniformity, which has excellent elasticity even if the rubber content is small, which also has excellent strength and further, which has an excellent appearance, an excellent coated appearance in particular. On the basis of finding of these, the present invention has been completed.